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.\" by Lennart Augustsson.
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.Dd July 17, 2025
.Dt SND_UAUDIO 4
.Os
.Sh NAME
.Nm snd_uaudio
.Nd USB audio and MIDI device driver
.Sh SYNOPSIS
.Cd "device sound"
.Cd "device usb"
.Cd "device snd_uaudio"
.Pp
In
.Xr rc.conf 5 :
.Cd kld_list="snd_uaudio"
.Pp
In
.Xr sysctl.conf 5 :
.Cd hw.usb.uaudio.buffer_ms
.Cd hw.usb.uaudio.default_bits
.Cd hw.usb.uaudio.default_channels
.Cd hw.usb.uaudio.default_rate
.Cd hw.usb.uaudio.handle_hid
.Cd hw.usb.uaudio.debug
.Sh DESCRIPTION
A USB audio device consists of a number of components: input terminals (e.g.\&
USB digital input), output terminals (e.g.\& speakers), and a number of units
in between (e.g.\& volume control).
.Pp
If the device supports multiple configurations, and there have been no
user-supplied values specified through the
.Xr sysctl 8
interface, the driver will select the best matching configuration supported by
the device during attach.
"Best" means the configuration with the most channels and highest quality in
sample rate and sample size.
.Pp
Refer to the
.Ql USB Audio Class Specification
for more information.
.Sh HARDWARE
The
.Nm
driver provides support for USB audio class devices and
USB MIDI class devices.
.Sh SYSCTL VARIABLES
The following settings can be entered at the
.Xr loader 8
prompt or in
.Xr loader.conf 5
and can also be changed at runtime with the
.Xr sysctl 8
command.
For a change to take effect during runtime, the device has to be re-attached.
.Bl -tag -width indent
.It Va hw.usb.uaudio.buffer_ms
Period of audio data processed at once, in milliseconds, from 1 to 8 (default
is 4).
Lower values mean less latency, but this can result in audible gaps due to
frequent CPU wakeups.
.It Va hw.usb.uaudio.default_bits
Preferred sample size in bits, from 0 to 32 (default is 0).
A value of 0 sets the sample size to the maximum supported sample size.
.Pp
Set this to select a smaller sample size if the device supports multiple sample
sizes.
.It Va hw.usb.uaudio.default_channels
Preferred number of sample channels, from 0 to 64 (default is 0).
USB 1.1 devices are limited to 4 channels due to bandwidth constraints, unless
a higher value is explicitly requested.
A value of 0 sets the sample channels to the maximum supported channel number.
.Pp
Set this to select a smaller channel number if the device supports multiple
channel configurations.
.It Va hw.usb.uaudio.default_rate
Preferred sample rate in Hz (default is 0).
If set to 0, the device's highest supported sample rate will be used.
.Pp
Note that if VCHANs are enabled, the sample rate will be overridden by
.Pa dev.pcm.%d.[play|rec].vchanrate
(see
.Xr sound 4 ) ,
which can also be used to adjust the sample rate during runtime.
.Pp
If
.Pa hw.usb.uaudio.default_rate
is non-zero,
.Pa dev.pcm.%d.[play|rec].vchanrate
will use it as its maximum allowed value.
.It Va hw.usb.uaudio.handle_hid
Let
.Nm
handle HID volume keys, if any (default is 1).
.Bl -tag -width 2n
.It 0
Disabled.
.It 1
Enabled.
.El
.El
.Pp
If
.Xr usb 4
has been compiled with
.Va USB_DEBUG
on, the following setting is also available:
.Bl -tag -width indent
.It Va hw.usb.uaudio.debug
Debug output level (default is 0).
.El
.Sh SEE ALSO
.Xr sound 4 ,
.Xr usb 4 ,
.Xr loader.conf 5 ,
.Xr loader 8 ,
.Xr sysctl 8
.Rs
.%T "USB Audio Class Specifications"
.%U http://www.usb.org/developers/docs/devclass_docs/
.Re
.Sh HISTORY
The
.Nm
driver first appeared in
.Fx 4.7 .
.Sh AUTHORS
This manual page was adopted from
.Nx 1.6
and modified for
.Fx
by
.An Hiten Pandya Aq Mt hmp@FreeBSD.org .
.Sh BUGS
The PCM framework in
.Fx
currently does not support the full set of USB audio mixer controls.
Some mixer controls are only available as
.Va dev.pcm.%d.mixer
sysctls.
